This application claims the benefit of Korean Application No. 00-59649, filed Oct. 11, 2000, in the Korean Patent Office, the disclosure of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a variable capacitor and a memory device employing the same, and more particularly to a variable capacitor capable of varying its capacitance, and a memory device capable of recording multiple values in unit memory cells by using the variable capacitor.
2. Description of the Related Art
Capacitors are circuit devices for storing electric energy, which are almost essentially employed in diverse electric circuits. In general, plural capacitors of varied types are used in an electric circuit.
Such capacitors are mainly classified as fixed capacitors and variable capacitors. Variable capacitors having large capacitances are generally used for mechanically varying the capacitances, which are mainly employed for filter circuits and power supplies.
Only fixed capacitors have been employed in conventional integrated circuits.
In general, integrated circuits manufactured through a semiconductor manufacturing process have respective electric devices reduced into minimized and optimized lithography scales in sizes in order to increase the integration efficiency. In such micro scales, forming highly precise micro patterns without errors on a semiconductor based on the existing lithography and etching processing technologies is extremely difficult. Accordingly, errors occur frequently in the manufacturing process with respect to the capacitances of fixed capacitors. Further, there exists a problem in that follow-up modifications or tuning can not be done with respect to the integrated circuit for apparatuses requiring precise capacitances if errors occur in the capacitances of capacitors manufactured through the semiconductor manufacturing process.
That is, since the integrated devices having lots of integrated electronic devices include capacitors formed therein of which capacitances can not be varied according to necessity, there exists another problem in that the defects of the integrated devices on the manufacture can not be cured as well as the problem that the integrated devices can not be compatibly used for a specification of an electronic unit.
In the meantime, in case of RAMs and flash memories as examples of the integrated circuits, they are constructed to store data in a binary value by means of one transistor and one capacitor.
Studies are steadily being conducted for increasing the data storage densities of such memory devices.
FIG. 1 is a cross-sectioned view for showing a conventional memory device.
Referring to FIG. 1, a memory device 10 has a transistor 12 formed on a substrate 11 and a capacitor 17. Reference numerals 18a, 18b and 18c indicate insulation layers.
The transistor 12 includes a source (S) 13, a drain (D) 14, and a gate (G) 15 which are spaced apart from each other.
The capacitor 17 is constituted with a first electrode 17a formed to be electrically connected to the source 13 through a conductive connection layer 16, a ferroelectric substance 17b, and a second electrode 17c. 
The memory device 10, as shown in the equivalent circuit of FIG. 2, receives write/read signals through the gate 15 connected to a control line 21, and charges the capacitor 17 to a predetermined voltage Vs and discharges a charged voltage through the drain 14 connected to a data line 22, to thereby write and read binary data.
The size of the memory cell should be reduced in order to increase the data storage density in such a conventional memory device. By the way, the size of the cell determining the data storage density is defined by variables such as a possible minimum pattern size (F) by the lithography, a clearance area (f) necessary for arrangements between cells, a minimum switching electric charge quantity Qswo, and the like.
In case of a DRAM as an example, the following relationship is obtained among a capacitance Co of the capacitor 17, a cell driving voltage Vdd, a switching electric charge quantity Qswo, and an area Ao(xcexcm2) of the capacitor 17.
Coxc3x9710xe2x88x9215xc3x97Vdd=Qswoxc3x97Aox1014xe2x80x83xe2x80x83{Formula 1}
The following Formula 2 is obtained from rewriting Formula 1.
Qswo=Coxc3x97Vdd/10xc3x97Aoxe2x80x83xe2x80x83{Formula 2}
Where, in case that Co=30fF and Vdd=3V are applied with respect to the respective variables, Qswo=9/Ao is obtained. Further, in case that a dielectric substance used for the capacitor 17 is a ferroelectric substance having the QSW of about 20 xcexcC/cm2, the area Ao of the capacitor 17 requires an area of about 0.23 xcexcm2. That is, it is difficult to form one side of the capacitor 17 less than 0.47 xcexcm.
Accordingly, the memory device 10 using the conventional fixed capacitor 17 has some restrictions in increasing the data storage density since difficulties exist in reducing the area of a unit cell storing data in a binary scale to an area less than a certain size.
The present invention is devised to solve the problems stated above, and it is an object of the present invention to provide a variable capacitor which can be manufactured by using semiconductor micro machining technologies and of which capacitance can be varied according to necessity.
It is another object of the present invention to provide a memory device in which data per cell can be stored in multiple values by applying the variable capacitor.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and, in part, will be obvious from the description, or may be learned by practice of the invention.
In order to achieve the above and other objects, the variable capacitor comprises a first electrode formed above a substrate; a second electrode installed to be able to float with respect to the first electrode and movable back and forth with respect to the first electrode; and a device to vary a capacitance, one end of which is connected to the second electrode and mounted with respect to the substrate to move the second electrode with respect to the first electrode in accordance with a voltage signal input through a driving electrode exposed externally.
Preferably, the device to vary the capacitance comprises a piezoelectric device contracted and expanded in response to a voltage input through the driving electrode.
Further, the variable capacitor comprises first and second electrodes formed at a distance spaced apart from each other above a substrate; a dielectric substance installed to move back and forth in a space between the first and second electrodes; and a device connected to the dielectric substance and formed above the substrate to move the dielectric substance back and forth in accordance with a voltage input through a driving electrode exposed externally, to vary a capacitance.
In order to achieve the above and other objects as stated above, the memory device according to the present invention comprises a transistor having a source, a gate, and a drain formed above a substrate, which are spaced apart from each other; and a variable capacitor connected to the source and having a device varying a capacitance of the capacitor. Note that any type of transistor may be used in the alternative including or having an emitter, collector, and a drain.
Preferably, the capacitor includes a first electrode formed above the substrate to be electrically connected to the source; and a second electrode formed to have a distance with respect to the first electrode which can be varied by the device to vary the capacitance.
The device to vary the capacitance includes a driving electrode extended externally; and an actuator mounted with respect to the dielectric substance, spaced apart by a predetermined distance along a direction opposite to the first electrode, and moving the second electrode back and forth in response to a signal input through the driving electrode, one end opposite to the first electrode being connected to the second electrode.
Further, the capacitor includes a first electrode formed on the substrate to be electrically connected to the source; a second electrode formed on the substrate to be spaced apart a distance from the first electrode; and a dielectric substance installed to move back and forth in a space between the first electrode and the second electrode, the device to vary the capacitance being formed above the substrate to move the dielectric substance back and forth.